Which of these numbers is composite? ${2,\ 15,\ 23,\ 31,\ 97}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 2, 23, 31, and 97 each have only two factors. The factors of 15 are 1, 3, 5, and 15. Thus, 15 is the composite number.